Add Tlon/Urbit channel plugin

Adds built-in Tlon (Urbit) channel plugin to support decentralized messaging on the Urbit network.

Features:
- DM and group chat support
- SSE-based real-time message monitoring
- Auto-discovery of group channels
- Thread replies and reactions
- Integration with Urbit's HTTP API

This resolves cron delivery issues with external Tlon plugins by making it a first-class built-in channel alongside Telegram, Signal, and other messaging platforms.

Implementation includes:
- Plugin registration via ClawdbotPluginApi
- Outbound delivery with sendText and sendMedia
- Gateway adapter for inbound message handling
- Urbit SSE client for event streaming
- Core bridge for Clawdbot runtime integration

Co-authored-by: William Arzt <william@arzt.co>

extensions/tlon/clawdbot.plugin.json

@@ -0,0 +1,11 @@
+{
+  "id": "tlon",
+  "channels": [
+    "tlon"
+  ],
+  "configSchema": {
+    "type": "object",
+    "additionalProperties": false,
+    "properties": {}
+  }
+}